1. Field
This patent specification relates to a method and apparatus for feeding sheet materials in general and, in particular, to such method adapted to separating and feeding sheet by sheet the uppermost out of sheet materials loaded on a sheet loading member, and an image forming apparatus incorporating such sheet feeding apparatus.
2. Discussion of the Background
As the methods previously disclosed for separating sheet by sheet the uppermost out of sheet materials loaded on a sheet loading member to be forwarded to an image forming unit, several methods are cited such as a corner presser separation method, separation pad method and bank separation method, for example.
Namely, the corner presser separation method is adapted to sheet separation with presser members by pressing both ends of leading edge of sheet material in the feeding direction, the separation pad method by pressing frictional members, and the bank separation method disclosed having a tapered face of fixed gate member to be collided with sheet materials to thereby be separated.
There cited among the methods are the separation pad method and the bank separation method, which offers advantages of relatively small in the number of parts and lower costs, being adapted to feeding various sheet materials different in size and thickness, e.g., post card, sealed letter and OHP sheet (Japanese Laid-Open Patent Applications No. 8-91612 and 10-139197).
In these known sheet separation methods, however, the former separation pad method has to include an additional measure for alleviating undue noises during image formation when applied to conventional less expensive and low speed reproduction machines in the range of 10 PPM or less (i.e., 10 or less copies of image formed per minute).
Since the undue noises are caused by sticking and slip movements of the sheet material passing through a nip portion between the feeding roller and frictional member, the feeding roller has to assume conventionally the half-moon shape to eliminate the noises.
This change in the roller shape places a limitation on the height for hosting the sheet loading plate. As a result, a pair of cylindrical collar members has to be additionally provided on both sides of the feeding roller having a diameter slightly smaller than that of feeding roller, whereby the number of parts is increased together with concomitant increase in manufacturing costs.
With increasing concern for resources and operation costs in recent years, recycled paper sheets have been used more often, in which the leading edges of sheet materials such as post cards and sealed envelopes are worn out and irregular, or having weld flash formed during sheet cutting steps, whereby conveyance load is unduly increased and non feeding situation may arise with relative ease in the separation pad method.
In addition, with the increase in reuse of the rear side of previously copied sheet, there caused are several difficulties such as increased scatter of frictional coefficient values between loaded sheets, to thereby causing multiple feeding and increased curling of the sheet caused by either fixing steps or environmental conditions, which results in undue load applied to leading edge of the sheet and failure in forwarding the sheet to separation unit, i.e., non feeding situation.
Furthermore, since the surface of the pad is brought into pressed contact to the feeding roller in the separation pad method, the angle between the pad and the forwarding direction of the sheet material (which corresponds to the angel of displacement for the loading base plate) has to be limited within a predetermined range.
The diameter of the feeding roller is therefore limited and the freedom of design layout for feeding apparatus is also limited, thereby causing another difficulty in reducing machine size, for example.
In the bank separation method as disclosed in Japanese Laid-Open Patent Application No. 8-91612, by contrast, the upper edge portion of the taper member is made flat and the nip portion with the feeding roller is relatively wide. As a result, the tapered face of the taper member is rather difficult to be provided within a certain range of placement angle.
Also, in the bank separation method for various sheet materials even largely different in size and thickness, it has been found from experiment, as detailed herein below, that satisfactory sheet feeding is feasible by equating modulus values to these sheet materials, which can be achieved by adjusting the distance in sheet forwarding direction between the point of pressed contact for the sheet material to feeding roller and the point of nip formation to be within a certain range (e.g., from 2 to 6 mm), and also adjusting the angle between the longitudinal direction of tapered face in taper member and sheet forwarding direction to be in a predetermined range (e.g., from 50° to 70°).
In order to achieve these conditions, however, the circumference of the feeding roller has to be large enough to be in contact with both of the noted contact points simultaneously, which results in a large diameter for the feeding roller and a concomitant increase in size of the sheet feeding apparatus as a whole, giving rise to another difficulty in reducing the size of feeding apparatus.
In addition, although the rotation of feeding roller is halted when the forwarding of a first sheet in the image forming unit is in progress, the following case has to be considered, in that the first sheet is still nipped between the feeding roller and an opposing gating member, and that the nipped sheet induces the concomitant rotation of the feeding roller through frictional force generated by the contact with a second sheet loaded on the base plate.
As a result, this concomitant rotation of the feeding roller then operates to forward the second sheet to be in contact with the tapered face of the taper member, when the tailing edge of the first sheet leaves the nip portion. If a friction coefficient between the second sheet and a further underlying sheet is smaller than that between the first and second sheets, the second sheet may climb over the tapered face to be forwarded further, thereby resulting the multiple feeding situation.
To obviate such difficulties, the present inventors have disclosed a sheet feeding apparatus capable of considerably reducing the effects of bending modulus coefficient for various kinds of sheet materials, and separating and subsequently forwarding sheet martial securely sheet by sheet without the non-feeding or multiple feeding (Japanese Patent Application No. 2001-217675)
The content of the above noted disclosure, however, is based primarily on previous experiences through various process of trial and error implemented to find optimum results on several factors such as the shape of the taper member, the direction of, and relative magnitude between, the forces in operation onto the system such as feeding pressure, separation pressure and others. Therefore, the theoretical analysis on the sheet feeding process has been earnestly awaited for.
It is therefore an object of the present disclosure to provide a method and apparatus capable of separating sheet materials securely without the non-feeding or multiple feeding through the clarification of conditions for obviating these undue feeding situations.